Exercise training may provide cardioprotection through increased coronary sensitivity to adenosine (ADO), a cardioprotective, hyperpolarizing vasodilator. The mechanism of enhanced sensitivity to ADO following EX is unknown, but may involve increased K channel activation. The long-term objective of this project is to determine the effect of EX on ADO stimulation of K currents in coronary smooth muscle (CSM) from different coronary vascular regions. Preliminary studies provide novel evidence that the K channel responsible for ADO-activated K current in resistance arteries is a large-conductance, Ca-activated K channel (KCa). Therefore, this project will determine by what mechanism KCa channels are activated by ADO in sedentary animals and following EX, and the arterial size dependence (conduit vs. resistance) of this effect. The overall hypothesis is that ADO activates KCa channels due to an increase in subsarcolemmal Ca independent of changes in bulk myoplasmic Ca due to Ca "unloading" from the SR. Furthermore, in SED, this activation occurs in resistance, but not conduit coronary arteries. Second, EX produces and "upstream shift" in the response to ADO, such that conduit arteries exhibit ADO-activated K current similar to resistance arteries from SED. In intact arteries, this enhanced ADO-stimulated K current will account, in part, for the enhanced vasodilation/relaxation following EX. The specific aims are to determine: 1) the effect of ADO and ADO analogs on K current in CSM from resistance and conduit arteries in SED and EX, 2) the type of K channel responsible for ADO-activated K current, 3) the role of G proteins, adenylate cyclase, and PKA in the activation of K current by ADO, 4) the role and source of subsarcolemmal Ca in KCa activation by ADO and 5) the contribution of K channels to the decrease in Cam and vasorelaxation following EX. Methods: Resistance (100-250 micron ID) and conduit (over 1 mm ID) arteries will be obtained from miniature swine following 16 wk of EX and compared to SED. K current and intracellular Ca will be determined by simultaneous voltage-clamp and fura-2 microfluorometry or high speed laser scanning confocal microscopy. Functional data will be obtained from simultaneous Cam and tension/diameter measures in intact arteries. The significance of the proposed study would be to; 1) provide the first direct evidence for, and mechanism of, KCa channel activation by ADO in CSM cells, 2) expand current information regarding the functional heterogeneity in the coronary vasculature to the subcellular level, and 3) provide a mechanism for enhanced coronary vasodilation to ADO after EX.